Tailoring a facile electronic and ionic pathway to boost the storage performance of Fe3O4 nanowires as negative electrode for supercapacitor application

Today, high-energy applications are devoted to boosting the storage performance of asymmetric supercapacitors. Importantly, boosting the storage performance of the negative electrodes is a crucial topic. Fe3O4-based active materials display a promising theoretical storage performance as a negative electrode. Thus, to get a high storage performance of Fe3O4, it must be tailored to have a higher ionic and electronic conductivity and outstanding stability. Functionalized graphite felt (GF) is an excellent candidate for tailoring Fe3O4 with a facile ionic and electronic pathway. However, the steps of the functionalization of GF are complex and time-consuming as well as the energy loss during this step. Thus, the in-situ functionalization of the GF surface throughout the synthesis of Fe3O4 active materials is proposed herein. Fe3O4 is electrodeposited at the in-situ functionalized GF surface with the crystalline nanowires-like structure as revealed from the various analyses; SEM, TEM, Mapping EDX, XPS, XRD, wettability test, and Raman analysis. Advantageously, the synthetic approach introduces full homogeneous and uniform coverage of the large surface area of the GF. Thus, Fe3O4 nanowires with high ionic and electronic conductivity are characterized by a higher storage performance. Interestingly, Fe3O4/GF possesses a high specific capacity of 1418 mC cm−2 at a potential scan rate of 10 mV s−1 and this value retained to 54% at a potential scan rate of 50 mV s−1 at an extended potential window of 1.45 V. Remarkably, the diffusion-controlled reaction is the main contributor of the storage of Fe3O4/GF electrode as revealed by the mechanistic studies.

1. Using the same deposition bath (100 mM FeCl 3 ), three electrodes were prepared using the linear sweep voltammetry (LSV) setup at potential windows: (2 to − 2 V), (2 to − 3 V), and (2 to − 4 V). 2. Using the same deposition potential window (2 to − 4 V), two other electrodes were prepared via the LSV technique using a deposition bath with various concentrations (150 mM, and 200 mM).3. Using cyclic voltammetry (CV) setup, all the prepared electrodes are activated in 1 M KOH solution via cycling the potential from 0 to − 1.45 V for 25 cycles at a potential sweep rate of 10 mV s −1 .
Note that: the best-optimized electrode (Fe 3 O 4 /GF) is the electrode that is prepared in a solution containing 200 mM FeCl 3 at a potential window (2 to − 4 V) .

Physical characterization
A field emission scanning electron microscope (SEM) (JEOL, Japan JSM IT-100) in conjunction with an energy dispersive X-ray spectrometer (EDX) device was conducted to probe the surface morphology and chemical composition of the modified electrodes.Transmission electron microscopy (TEM) and high-resolution electron microscopy (HR-TEM) (JEM-HR-JEOL-JEM 2100, Japan) were conducted for the investigation of the material's morphology and the material's crystal structure.X-ray photoelectron spectroscopy (XPS) was also probed to disclose the electrodes' surface composition (using monochromatic X-ray Al K-alpha radiation, Thermo Fisher Scientific).To further analyze the crystalline structure, X-ray diffraction (XRD) with Cu K α radiation, STOE STADI, was used.Furthermore, the Attention Biolin Scientific (Version 2.7) wettability test was probed to explore the contact angle of the prepared materials, i.e., surface hydrophilicity.Moreover, Raman spectra captured by Lab RAM HR Evolution were employed to monitor the presence of the redox-active components and the level of surface modification of the GF support.

Electrochemical measurements
The storage performance of the synthesized active materials was determined from the relevant electrochemical measurements operated using a Biologic Potentiostat (VSP-300) at room temperature (25 °C ± 1) and electrolyte solution containing 1 M KOH.electrochemical tests: CV, galvanostatic charge-discharge (GCD, and electrochemical impedance spectroscopy (EIS) (at open circuit potential from 100 kHz to 10 mHz) measurements were carried out in three-electrode cell (Fe 3 O 4 /GF is the working electrode, graphite rod is the counter electrode, and SCE is the reference electrode) to evaluate the storage performance of the as-synthesized materials.
The C s calculated by Eq. 1 and 2 from the integrated area under the CV and GCD curves, respectively 33,35,36 : where C s is the specific capacity, ∫IdV/v or ∫Idt is the total charge, S is the electrode geometric surface area (GF (6 faces) has S = 0.712 cm 2 ), ∫IΔt is the total charge, and Δt is the discharge time.

Optimization of GF surface alternation and Fe 3 O 4 deposition
Herein, the multifunction one-step method is proposed to synthesize Fe 3 O 4 in the crystalline nanostructure over the in-situ functionalized GF surface with high areal capacity.The GF functionalization and Fe 3 O 4 electrodeposition are done by sweeping the potential from the oxidative potentials (positive potentials) followed by the reductive potentials (negative potentials) via the LSV technique, respectively, in the deposition bath containing FeCl 3 .While the role of the oxidative potentials is to functionalize the GF surface, the role of the reductive potentials is to deposit the Fe 3 O 4 .Functionalization of hydrophobic GF surface as seen from Fig. 1 occurred at the oxidative potentials with the aid of the formed oxygen, H 2 O, and ClO 3 − species that are in-situ formed at high positive potential 32,[37][38][39] .The inserted oxygenated functional groups at the GF surface were probed by various surface characterization tools, particularly XPS studies (c.f. Figure 3).Additionally, the in-situ functionalized GF assisted in the homogenous uniform deposition of Fe 3 O 4 redox active material around almost all GF fibers (c.f. Figure 5).The formation of the Fe 3 O 4 is started in the reductive region by the reduction of Fe 3+ to Fe 2+ followed by the formation of Fe 3 O 4 assisted by the hydrogen evolution reaction at the electrode surface according to the following Eqs.(3-5) 39,40 : (1) www.nature.com/scientificreports/Thus, according to the above-mentioned idea, the conditions were optimized by investigating the width of the potential window together with the electrolyte concentration.Firstly, three electrodes were prepared, using a concentration of the deposition bath equal to 100 mM FeCl 3 , applying various potential windows (2 to − 2 V, 2 to − 3 V, and 2 to − 4 V), see Fig. 2A.Importantly, the amounts of the loaded Fe 3 O 4 are increased by opening the potential to the higher negative values.As a result, and from Fig. 2B, the electrode that is obtained at the extended potential window (+ 2 to − 4 V) is the best-prepared electrode due to the integrated surface area under the CVs resulting in a higher areal capacity (see Fig. 2C).Further, the effect of the FeCl 3 concentration at the optimized deposition potential window is investigated using two other concentrations, 150 and 200 mM.Advantageously, as the deposition bath concentration is increased, the level of the functionalization is enhanced, and this is clear from the positive current's region of all electrodes that is assisting in depositing the active materials in a uniform and homogeneous manner utilizing the maximum available GF surface area (see Fig. 2D).Also, the degree of the deposition of the Fe 3 O 4 active materials is increased by extending the potentials to the higher reductive potentials, see Fig. 2D.Accordingly, the electrode that is prepared at 200 mM and wide potential window is the best and this is notable from the area under CVs and the calculated areal capacity as shown in Fig. 2E,F.Therefore, the electrode prepared using 200 mM FeCl 3 solution and applying a potential window from + 2 to − 4V (Fe 3 O 4 / GF) is selected as the best electrode for further physical characterizations and electrochemical measurements.

Physical characterization
XPS analysis is utilized to confirm the existence of Fe in the form of Fe 3 O 4 , and the incorporation of the oxygenated functional groups at the GF surface.From Fig. 3, the incorporation of the Fe element during the synthesis process is clear from the existence of the extra peak around 712 eV in comparison with the survey of pure GF surface.Also, the approval of the Fe 3 O 4 preparation is verified by the existence of the two oxidation states of iron (Fe 2+ and Fe 3+ ).While the existence of Fe 2+ is proven by the appearance of two peaks around 711 and 725 eV that are ascribed to the Fe 2p 3/2 and Fe 2p 1/2 , respectively, the existence of the Fe 3+ is proven by the appearance of the peaks around 713 and 728 eV that are assigned to the Fe 2p 3/2 and Fe 2p 1/2 , respectively (see Fig. 3) [41][42][43] .Additionally, from O 1s spectra as displayed in Fig. 3, the formation of Fe 3 O 4 is confirmed by the appearance of a peak assigned to the Fe-O bond 44 .Also, from O 1s spectra, one concludes that the successful in-situ GF functionalization is confirmed by displaying more oxygenated functional groups compared to the pristine GF surface.Additionally, the percentage of the O in the Fe 3 O 4 /GF electrode is 4 times that of the GF surface indicating the massive deposition and the in-situ functionalization.Further from C 1s, the appearance of the hump at high binding energy around 288 eV indicates the oxidation of the GF surface, and this is clear from the deconvoluted C 1s compared to that of the GF surface 32,35,45 .
(5) and D bands vanish, indicating that there is an excessive amount of Fe 3 O 4 deposited that completely encases the GF fibers (see Fig. 4A).As a result of the above analysis that confirms the in-situ GF surface fluctuations and heavy deposition of the Fe 3 O 4 materials, the Fe 3 O 4 /GF electrode shows exceptional hydrophilicity.As displayed in Fig. 4B, the contact angles of GF and Fe 3 O 4 /GF electrodes are 174.29°and 0°, respectively.This improves the capacity performance by smoothing the electrolyte ions' diffusion paths for the reaction with redox-active materials.
XRD analysis is done and boosted the above results.Firstly, from Fig. 4C, the formation of the Fe 3 O 4 is established by obtaining peaks around 35°, 42°, 52°, 58°, 62°, and 82° assigned to the crystalline structure of Fe 3 O 4 (JCPDS No. 19-0629) 47,48 .Secondly, as consistent with Raman analysis, the diffraction peaks assigned to the graphitic structure are not present in the Fe 3 O 4 /GF electrode, showing that the active materials have completely covered the fibers of GF with high thickness confirming the massive and homogeneous deposition.
The surface morphology and elements distribution of GF and Fe 3 O 4 /GF electrodes are investigated by SEM and mapping EDX analysis, respectively.From Fig. 5, GF demonstrates a fiber's flawless surface with little white spots that represent carbon dust after manufacture 34,49 .Whereas Fe 3 O 4 /GF, as shown in Fig. 5, exhibits uniform encasement of Fe 3 O 4 on the in-situ altered GF surface in nanowire shape.Uniform encasement indicates effective surface modification via inserting oxygenated functional groups during the preparation of Fe 3 O 4 /GF which compiles with the findings obtained from XRD and Raman analysis.The complete encasement of GF fibers by active materials and the extraordinary hydrophilicity facilitates the electrolyte ions' pathways toward the active sites predicting a high C s (c.f. Figure 8).Moreover, mapping EDX of Fe 3 O 4 /GF demonstrates the massive deposition www.nature.com/scientificreports/ of the Fe 3 O 4 by obtaining a massive amount of Fe element, high O percent, and low C percentage indicating the complete encasement that is consistent with the previous analyses (see Fig. 5).TEM and HR-TEM analyses are conducted for Fe 3 O 4 /GF electrode for further confirmation of morphology and crystallinity nature of Fe 3 O 4 active material.From Fig. 6, TEM images at different magnifications (Fig. 6) display a nanowire morphology with a small diameter (see Fig. 6B) that is consistent with SEM analysis.Furthermore, the Fe 3 O 4 active particles' selected area electron diffraction (SAED) is shown in Fig. 6D which indicates the polycrystalline nature of Fe 3 O 4 due to the presence of bright spots surrounding the rings that are consistent with XRD analysis 50,51 .

Electrochemical storage performance of Fe 3 O 4 /GF electrode
The peak intensities and the diffusion accessibility of the electrolyte ions to the underneath layers of the active materials vary throughout the first several cycles of operation.Thus, the electrochemical activation must be done to reach a steady state before the evaluation of the electrochemical storage performance of the Fe 3 O 4 active materials 52,53 .From Fig. 7A, there are characteristic peaks of Fe 3 O 4 active materials at the cathodic branch (I, and II) and three peaks at the anodic branch (III, IV, and V).During the cathodic direction, the Fe 3+ is electrochemically reduced to Fe 2+ (peak I) and then to Fe 0 (peak II).Moving towards the anodic direction, i.e., toward high potential values, the three characteristics of anodic peaks appeared due to; (a) formation of Fe(OH) by the adsorption (peak III); (b) electrochemical oxidation of the Fe 0 to the Fe 2+ ; and (c) overcharging of the Fe 2+ to Fe 3+52,53 .As shown in Fig. 7A, while the peak intensity of II, III, and IV is decreased by repetitive cycling, the intensity of peaks I and V is increased with cycling.Thus, the activation step is done to reach a steady state and nearly the same behavior, i.e., the same areal capacity (see Fig. 7B).
After the electrochemical activation, the electrochemical storage performance of the Fe 3 O 4 /GF electrode is studied utilizing the CV, GCD, and EIS measurements.As seen from Fig. 8A, CVs of Fe 3 O 4 /GF and GF electrodes show that the value of capacity is not significantly influenced by the GF support.However, the electrochemical change of Fe between its oxidation states, as indicated by the peaks in the anodic and cathodic scan, results in a CV of Fe 3 O 4 /GF (red line) with a large surface area indicating a higher capacity, and this reflected the 3-D porous structure of GF support, complete encasement of GF fibers by redox-active Fe 3 O 4 , and the exceptional ionic conductivity of the prepared electrode.The C s of the Fe 3 O 4 /GF is calculated using Eq. 1 and displays a value  1).Additionally, by using Eq. 1 and normalizing capacity by the loaded mass (9.3 mg cm −2 ) instead of area, the Fe 3 O 4 /GF electrode displays a specific capacity of 153 C g −1 which is also higher than other reported values, e.g.Fe 3 O 4 /SDS 54 , CNT/Fe 3 O 4 55 , Fe 3 O 4 nanoparticles 56 .Interestingly, one of the significantly effective parameters for the application and performance of SCs is the operational voltage window.Fe 3 O 4 /GF electrode exhibits an extended voltage window up to 1.45 V which is one of the widest potential windows obtained for the storage performance of Fe 3 O 4 materials in the negative potential region (see Table 1).Additionally, from Fig. 8B, the C s value of the Fe 3 O 4 /GF electrode is calculated at various potential scan rates.Figure 8C displays the variation of C s and the obtained values are 1418.3,1160.7,1044.2, 968.4,909.6, 825.6, and 769.6 mC cm −2 at various potential scan rates of 10, 15, 20, 25, 30, 40, and 50 mV s −1 , respectively.The stability test is one of the most crucial factors in determining how well the SC material performs.The stability of Fe 3 O 4 is shown in Fig. 8D for 200 cycles at a potential sweep rate of 20 mV s −1 .Fe 3 O 4 /GF exhibits limited stability behavior, which is explained by the mechanical stress that results from the electrolyte ions' intercalation and deintercalation within the active materials during the charge transfer process that occurs throughout the charging and discharging process.
Interestingly, the origin of the storage mechanism of the synthesized electrode can be investigated by the analysis of the CVs at various potential sweep rates.Therefore, the Trasatti method is introduced to calculate the percentage of the contribution of the different storage processes of the synthesized materials using Eqs.6 and 7 5,6,65 : where q represents the charge (C) at various potential scan speeds (ν), a and b are constants, q ∞ denotes the charge owing to the surface process (fast surface faradaic and adsorption of electrolyte ions), and q t is the total charge (bulk and surface processes).The values of q ∞ and q t are 0.362 and 1.36 C, respectively, based on the intercept values of Fig. 9A,B.The ratio of the surface process (surface faradaic and ions adsorption) to the overall charge is 26.6% suggesting the combination storage mechanism during the charging and discharging of Fe 3 O 4 / GF electrode materials.But the Trasatti method gives this ratio based on the extrapolation of q ∞ and q t at the ν equal ∞ and 0, respectively.Thus, the storage mechanism of the active materials over the selected potentials sweep rates is mainly dependent on the intercalation and de-intercalation within the active materials and this is clear from the appearance of various redox peaks of CVs of Fe 3 O 4 /GF.Also, this behavior is confirmed by change separation using the Dunn method [66][67][68] .Figure 9C displays that the percentage of bulk faradic process is the main percentage of storage mechanism.Interestingly, at high-speed scan (50 mV s −1 ) the bulk faradic reaction contributes by 67.2%.
Furthermore, the bulk faradaic process may be further analyzed by using the relation concerning redox peak current and the potential scan rate using Eq. 8 69 : (6) q = q ∞ + aν −0.5 (7) where I p is the redox peak current, v is the potential sweep rate, and a and b are constants.In this instance, the b-value represents the rate at which the electrolyte ions inside the active materials intercalate and de-intercalate throughout the charging and discharging process.For the diffusion-controlled process (battery behavior), the b-value is equal to 0.5, while for the intercalation pseudocapacitive, it is equal to 1. Bot mechanism, i.e.Bulk faradaic and intercalation pseudocapacitive, is suggested when the b value is between 0.5 and 1.The b-value, which is ascribed to the relation between the redox current of peaks I and V (from Fig. 7A) with the potential scan rate, was obtained from the slopes of Fig. 9D,E and are found to be 0.67 and 0.59, respectively.One can conclude from b values that the bulk faradaic process is mixed between intercalation and diffusion-limited mechanism.However, the predominant mechanism is the diffusion-controlled process due to the values being close to 0.5.Secondly, the GCD test is obtained for Fe 3 O 4 /GF at various current densities.Figure 10A shows that the shape of the GCD curve is a battery-like electrode by displaying the plateau which is consistent with the data and results obtained from the investigation of CVs of the Fe 3 O 4 /GF electrode.Figure 10B  www.nature.com/scientificreports/constant phase element concerning double layer capacitance (CPE 1), charge transfer resistance (R ct ), and constant phase element concerning the capacity of (CPE 2)) 70,71 .Interestingly, the encasement of Fe 3 O 4 active materials around GF fibers makes the ESR of GF increase resulting in a decrease in the total electronic mobility.However, compared to the GF electrode, the Fe 3 O 4 electrode has a lower ESR value.This suggests that the preparation method improves the GF material's electronic conductivity.The admittance plot (see Fig. 10E) provides additional confirmation of this.As a consequence of the Fe 3 O 4 /GF electrode's electrochemical studies above, which are conducted as a negative electrode for SC applications.the Fe 3 O 4 /GF electrode compared to other related Fe 3 O 4 -based materials shows distinct features.The characteristics and features of the prepared electrode materials are concise as follows: (i) it is prepared with just one inexpensive, and simple electrochemical procedure, (ii) binder-free method, (iii) in contrast to the other work, where a pretreatment step is used, the GF is in-situ altered, improving both ionic and electronic conductivity, and (iv) Fe 3 O 4 /GF has an exceptionally broad potential window for operation (1.45 V).However, the prepared electrode suffers from limited stability due to deterioration of the swelling and shrinking of the electrode materials throughout the cyclic performance which may be improved in future work by addressing many parameters such as the incorporation of other metals to develop a binary system that can relieve the stress during the cyclic performance, and coating the active materials with the above layer from carbon material that is increasing the stability without affecting the other parameters.

Conclusion
The easy synthesis of Fe 3 O 4 nanostructure with distinctive storing performance is the focus of this work besides the investigation of its storage mechanism.The proposed method has many advantages over the other reported methods for the preparation of the Fe 3 O 4 at the surface of the modified carbon support.Both the in-situ surface alteration of GF and Fe 3 O 4 nanowires deposition occurred in a single simple procedure that was energy conservative, inexpensive, safe, and time-efficient.Various analyses were utilized to emphasize the deposition of the nanostructure of the prepared materials at the in-situ altered GF surface.Advantageously, inserting the functional groups at the GF surface provides a homogenous encasement of Fe 3 O 4 around GF fibers and accelerates the contact of the active materials with electrolyte ions.Interestingly, the Fe 3 O 4 /GF electrode has a unique storage performance over a wide potential window up to 1.45 V, which is considered one of the widest potential windows for the Fe 3 O 4 in the negative potential region, especially in the alkaline medium.Fe 3 O 4 / GF electrode displayed a specific capacity of 1418 mC cm −2 at a potential scan rate of 10 mV s −1 and this value retained to 54% at a potential scan rate of 50 mV s −1 over an extended potential window of 1.45 V.The higher storage performance is attributed to the high deposited material over the large GF area, high accessibility of ions, and enhanced electronic mobility.Furthermore, the mechanistic investigation reveals that the quick surface redox reaction and the diffusion-limited interaction between the electrolyte ions and the Fe 3 O 4 redox active sites combine to form the produced electrode's storage mechanism.However, the predominated mechanism is the limited diffusion reaction.Thus, this electrode can be used efficiently with the positive electrode that is based on the capacitive operation to make an efficient asymmetric SC device with a wide potential window.

Figure 6 .
Figure 6.HR-TEM images with various magnifications (A-C), and (D) the corresponding SAED pattern of the Fe 3 O 4 /GF electrode.

Figure 7 .
Figure 7. (A) CVs of Fe 3 O 4 /GF electrode before (black line) and after (red line) the electrochemical activation.(B) The variation of the specific capacity with cycle number during the electrochemical activation of the Fe 3 O 4 / GF electrode.

Figure 9 .
Figure 9. Trasatti method for the surface (A) and bulk reaction (B) of Fe 3 O 4 /GF electrode.Dunn method for determining bulk contribution at a potential scan rate of 10 mV s −1 (C).The variation of log(Ip) with log(v) of I (D) and V (E) peaks (from Fig. 7A) of the Fe 3 O 4 /GF electrode.

Figure 10 .
Figure 10.(A) GCD curves of Fe 3 O 4 /GF electrode at various current densities.(B) The variation of C s of Fe 3 O 4 / GF electrode with various current densities.(C) Nyquist plots of GF and Fe 3 O 4 /GF electrodes at OCP (D) Nyquist for the Fe 3 O 4 /GF electrode.(E) Admittance plots of GF and Fe 3 O 4 /GF electrodes at OCP. 1234567890)